TTF1 and small cell lung carcinoma: When the expression of two proteins used clinically in the diagnosis of SCLC was examined, thyroid transcription factor 1 (TTF1 of NKX2-1 gene) expression was, interestingly, reciprocal to NEUROD1 expression in Group I cells while synaptophysin (SYP) expression appeared to parallel that of INSM1 (Figure 2F).